The present invention relates to FM stereo, and more particularly to a method and apparatus for reproducing FM stereo sounds in a manner to reduce noise and distortion, while preserving the quality of the reproduced stereo sounds.
It is well known that when FM radio transmission is reproduced in the mono mode, there is a relatively high quality of sound reproduction, with little background noise and relatively little distortion. However, it is also well known that when there is FM transmission of a stero signal, there is much greater noise and distortion. More specifically the L-R sound is quite susceptible to multipath distortion. This occurs when the FM signal is reflected off the side of a building, hill, or even by a bridge, so that there is a second or third delayed signal reaching the receiver. When these delayed signals are superimposed on the main signal transmitted directly to the receiver, there is distortion in the combined signal that is reproduced.
The common method of producing FM stereo is to transmit an L+R signal (which is a combination of the left stereo signal and the right stereo signal) as a frequency modulation on the radio frequency carrier, which, when demodulated, has signals in the band from 0 to 15 kHz. There is also an L-R signal component (which is the difference between the left stereo signal and the right stereo signal), and this is transmitted in a band width which is centered on a 38th kHz subcarrier, with the range of this band being from about 23 kHz to 53 kHz. These two sets of signals are separated in the receiver, then fed to a decoding matrix which combines these signals so as to provide separate L and R outputs that correspond to the original left and right stereo signals, and are in turn fed to left and right speakers to produce the stereo sound.
The L+R signal is, of itself, of high quality, while most of the random noise and multipath distortion is attributable to the L-R signal. There have been attempts in the prior art to somehow mask or eliminate the random noise and distortion in the L-R signal. It has been recognized in the prior art that most of the undesirable noise and distortion is present in the higher frequency range of the L-R signal, and for this reason, one approach has been to mute the high frequency part of the L-R signal component, particularly where there is low signal strength so that the noise and distortion is more noticeable. Such an approach is disclosed in U.S. Pat. No. 3,943,293, Bailey. However, by so muting or eliminating the L-R information, much of the stereo effect is lost, with the L+R signal being transmitted from both speakers so that, in effect, there is produced a more mono-like sound rather than a full stereo sound.
The overall effect is that when the music is playing loudly, the sound moves out to the speakers for the stereo effect. However, when the music begins to be less loud, it moves from the location of the speakers to a center location. Likewise, when a single instrument is playing loudly, it appears to come from the speaker location, but then move to a location between the speakers when that same instrument is playing more softly.
Another consideration is that many of the prior art approaches have failed to recognize the significance or role of what might be termed the "ambient" sounds in a stereo recording. The quality of a stereo recording is affected by sounds which are reflected at the recording location and then picked up by the recording microphones. While these sounds may or may not be directional, when added to the main musical sound, they produce a certain fullness of the sound which is more representative of a live performance. Such reflected or delayed sounds are the "ambient" sounds referred to above.
In view of the above, it is an object of the present invention to reproduce FM stereo sound in a manner to minimize random noise and distortion, while preserving the directional information and fullness of the original stereo signal.